Pulleys or bearing blocks are employed on a wide variety of sailing craft and are also used in various other applications, in order to change the direction of a moving line, or to provide a mechanical advantage in a tackle system.
A conventional pulley comprises a head having a swivel post for attachment to a support, a pair of side plates extending in parallel in a spaced relation from the head, and a sheave rotatably mounted between the side plates. The sheave has a continuous outer circumferential groove upon which a line is received.
Also, most conventional pulleys typically use some type of main bearing system to reduce rolling friction due to forces imposed by the line generally perpendicular to the axis of rotation of the sheave, generally referred to as radial loads. The bearings may include roller, ball or sleeve bearings. These bearings are usually disposed between a fixed inner race secured between the side plates and the outer rotatable sheave.
It is also known, especially in higher capacity blocks, to employ additional thrust bearings between the sides of the sheave and the side plates to prevent rubbing of the sheave against the side plates during rotation. For example, a circulation groove may be provided in the annular side surfaces of the sheave, and a plurality of ball bearings are loosely disposed in the grooves. Portions of the ball bearings project beyond the annular surface and can come into engagement with the side plates or other bearing surface secured to the inner race when thrust loads are imposed on the sheave.
The thrust bearings described above have several disadvantages. First, since the entire circumference of the groove is filled with ball bearings, the number of bearings employed may be very high in comparison to the expected load, and assembly and disassembly of the block can be very tedious. For example, when the block is disassembled to inspect or replace the main bearings, the thrust bearings tend to fall out and get lost.
Another drawback is that the material required to provide grooves in the sheave adds significant mass and radial thickness at the outer rim of the sheave. When bearing blocks are used on sailing vessels, it is important that the line engaged with the sheave should be capable of being released quickly, requiring rapid acceleration of the sheave. Since acceleration is inversely proportional to mass, any reduction of mass in the sheave would be greatly beneficial. Also, it would be desirable to decrease the radial thickness at the outer rim of the sheave in order to allow use of additional main bearings and increase load capacity.